Catherine J. McMahan

RAG Reexpression and DNA Recombination at T Cell Receptor Loci in Peripheral CD4+ T Cells

Under most circumstances, allelic exclusion at the T cell receptor (TCR)beta locus is tightly regulated. Here, we describe a system in which TCRbeta allelic exclusion is overcome as a result of V(D)J recombination in peripheral CD4+ T cells. In TCRbeta chain transgenic mice, tolerogen-mediated chronic peripheral selection against cells expressing the transgene leads to surface expression of endogenous TCRbeta chains. Peripheral CD4+ T cells reexpress the recombination activating genes, RAG1 and RAG2, and contain signal end intermediates indicative of ongoing V(D)J recombination. The rescue from deletion of mature T cells expressing newly generated TCRbeta chains suggests that receptor revision plays a role in the maintenance of peripheral T cell tolerance.

Lymphocytes rearrange, edit and revise their antigen receptors to be useful yet safe

Pamela Fink and Catherine McMahan discuss how B and T cells test for useful antigen receptors and weed out potentially harmful ones, with special attention paid to T-cell receptor revision, a newly described mechanism by which mature T cells can maintain self tolerance.

Receptor Revision in Peripheral T Cells Creates a Diverse Vβ Repertoire

In Vβ5 transgenic mice, the age-dependent accumulation of Vβ5−CD4+ T cells expressing endogenous Vβ elements represents an exception to the rule of strict allelic exclusion at the TCRβ locus. The appearance of these cells is limited to the lymphoid periphery and is driven by a peripherally expressed tolerogen. Expression of the lymphoid-specific components of the recombinase machinery and the presence of recombination intermediates strongly suggest that TCR revision rescues tolerogen-reactive peripheral T cells from deletion. Here, we report that the appearance of Vβ5−CD4+ T cells is CD28-dependent. In addition, we find that the TCR repertoire of this unusual population of T cells in individual Vβ5 transgenic mice is surprisingly diverse, both at the level of surface protein and at the nucleotide level within a given family of V(D)Jβ rearrangements. This faithful recreation of the nontransgenic repertoire suggests that endogenous Vβ-expressing populations do not arise from expansion of an initially rare subset. Furthermore, the undersized N regions in revised TCR genes distinguish these sequences from those generated in the adult thymus. The diversity of the revised TCRs, the minimal mouse-to-mouse variation in the expressed endogenous Vβ repertoire, the atypical length of junctional sequences, and the CD28 dependence of the accumulation of Vβ5−CD4+ T cells all point to their extrathymic origin. Thus, tolerogen-driven receptor revision in peripheral T cells can expand the TCR repertoire extrathymically, thereby contributing to the flexibility of the immune repertoire.

T Cell Receptor Revision Does Not Solely Target Recent Thymic Emigrants

CD4+Vβ5+ T cells enter one of two tolerance pathways after recognizing a peripherally expressed superantigen encoded by an endogenous retrovirus. One pathway leads to deletion, while the other, termed TCR revision, results in cellular rescue upon expression of an alternate TCR that no longer recognizes the tolerogen. TCR revision requires the rearrangement of novel TCR β-chain genes and depends on recombinase-activating gene (RAG) expression in peripheral T cells. In line with recent findings that RAG+ splenic B cells are immature cells that have maintained RAG expression, it has been hypothesized that TCR revision is limited to recent thymic emigrants that have maintained RAG expression and TCR loci in a recombination-permissive configuration. Using mice in which the expression of green fluorescent protein is driven by the RAG2 promoter, we now show that in vitro stimulation can drive reporter expression in noncycling, mature, peripheral CD4+ T cells. In addition, thymectomized Vβ5 transgenic RAG reporter mice are used to demonstrate that TCR revision can target peripheral T cells up to 2 mo after thymectomy. Both sets of experiments strongly suggest that reinduction of RAG genes triggers TCR revision. Approximately 3% of CD4+Vβ5+ T cells in thymectomized Vβ5 transgenic reporter mice have undergone TCR revision within the previous 4–5 days. TCR revision can also occur in Vβ5+ T cells from nontransgenic mice, illustrating the relevance of this novel tolerance mechanism in unmanipulated animals.

Modulation of TCRβ Surface Expression During TCR Revision

TCR revision is a tolerance mechanism by which self-reactive TCRs expressed by mature CD4(+) peripheral T cells are replaced by receptors encoded by genes generated by post-thymic DNA rearrangement. The downmodulation of surface TCR expression initiates TCR revision, and serves as a likely trigger for the induction of the recombinase machinery. We show here in a Vβ5 transgenic mouse model system that downregulation of the self-reactive transgene-encoded TCR is not maintained by transgene loss or diminished transcription or translation. The downregulation of surface TCR expression likely occurs in two stages, only one of which requires tolerogen expression.